ZOOLOGY. 



minute pores at the surface, passes along radiating tubes (in- 

 current channels) to the central cavity (cloaca) and escapes 

 through one or more larger openings (oscula) at the unat- 

 tached end. There is no true ccelom (see 56). 



2. Parts are arranged about the central cavity but not usu- 

 ally in a symmetrical fashion. 



3. There are two distinct layers, ectoderm and entoderm. 

 These are separated by a gelatinous mass in which are in- 

 cluded cells of different kinds (mesenchyma or mesoderm) 



FIG. 74- 



FIG. 74. Leucandra, a simple type of sponge. (From Delage and Herouard; " Traite 

 de Zoologie Concrete.") 



Questions on the figure. What is the position of the osculum ? Which 

 is the attached end? How many individuals are represented in the cut? 



FIG. 75. Diagrams to illustrate the development of one of the simpler types of 

 sponge: i, the egg; 2, section of 16- to 32-celled stage; 3, section of later stage, a 

 ciliated larva (blastula) ; 4, gastrula; 5, section through older larva which has become 

 attached by the end containing the blastopore. New openings break through by the 

 coalescence and perforation of the ectoderm and entoderm, and a form results such 

 as is figured in Fig. 76. a, archenteron; bl., blastopore; ec., ectoderm; en., entoderm; 

 mes., mesenchyma; s, segmentation cavity. 



Questions on the figures. What terms would be applied to the cleav- 

 age and gastrulation in this sponge? What is suggested as to the mode of 

 forming mesoderm? The attachment of the sponge by the blastopore end 

 of the larva necessitates what later development? See Fig. 76. 



